Abstract
Ten infectious bronchitis viruses (IBVs) were isolated from diseased chicken flocks in China between 2018 and 2024. Phylogenetic, homology, and pairwise comparisons of the complete S1 gene revealed that the 10 isolates shared a close genetic relationship with the European L1148 vaccine strain. Complete genomic sequence of isolate I0317/22 confirmed this result. Inoculation of 1-day-old specific pathogen free (SPF) chickens with I0317/22 induced mild clinical signs. The virus replicated at low levels in most of the tissues, except in the respiratory and upper digestive (preventriculus) tracts. We speculate that the 10 isolates derived from the L1148 vaccine strain and spread with low pathogenicity within chicken flocks. In 1-day-old SPF chickens, inoculation with I0317/22 and vaccination with the Mycoplasma gallisepticum (MG) F vaccine strain demonstrated that the MG F vaccine did not promote I0317/22 replication. Similarly, challenge of the chickens inoculated with MG F after 28 days with strain I0305/19 did not enhance I0305/19 replication compared to the control group. However, inoculation with MG F suppressed the neutralizing antibody responses against I0317/22 and I0305/19. We hypothesize that MG F strain induced suppression of neutralizing antibodies by affecting the host immune response to IBV infection, thereby increasing susceptibility to other pathogens, such as Escherichia coli. This likely led to mixed infections where synergistic interactions between pathogens exacerbated disease severity beyond what would be expected under experimental conditions alone.